1. Field of the Invention
The invention relates to the xerographic recording and data storage arts and more particularly concerns generation of frost image patterns on a surface of a thermoplastic medium charged to a potential level below the normal frost formation potential.
2. Description of the Prior Art
In one commonly practiced form of electrostatic recording, an electrostatic charge pattern forms a latent image of the data or the object whose image is to be recorded on the surface of an insulating medium and is then made visible. The charge pattern may be generated by direct electrical charge deposition such as by the simple process of irradiation of the insulator surface by electrons flowing through a stencil. The more usual process involves the cooperative action of an electric field and a pattern of different shades of light projected onto the surface of an insulating photoconductor layer. The latent image thus formed is then rendered visible by deposition of fine electroscopic developer particles which adhere only where the charges reside. The visible powder image, once formed, is fixed or developed permanently on the surface of the photoconductor medium, or the particles forming the powder image may be transferred intact to a record medium where they then are developed and made permanent. Only when the powder image is actually transferred is the photoconductor medium re-usable.
A more recent kind of electrostatic recording involves deformation of a thermoplastic medium whereby permanent or erasable recording may be effected according to two distinct methods now universally known as the relief and the frost methods, respectively. While both of these methods generate image patterns by deformation of a thermoplastic material in response to electrostatic forces located in a latent charge surface image, the relief imaging method enjoys only specialized use. Image formation by the relief method depends upon the presence of significant electrostatic gradients; thus, a single line deformation may be generated along a locus defined by a steep charge gradient and relief imaging therefore does not occur where there is a uniform of slowly changing charge distribution.
On the other hand, the frost type of thermoplastic recording in its simplest form provides on uniformly charged areas a uniform distribution of relatively very small random surface wrinkles that scatter light and are best described as having a frosted appearance. Frost images are readily projected or read out in contrast to relief images, which are characterized as inherently storing phase data and require complex read-out optics of the Schlieren type. The present invention is therefore directed to an improvement in relatively less complex prior art frost thermoplastic recording methods.
In the usual frost method of imaging on thermoplastic material, a latent-image-defining electrostatic charge pattern is formed on a heat-softenable insulating film depending upon the use of a suitable photoconductive layer lying under the deformable film. Typical processing steps applied to this photoreceptor configuration involve a charging step, an optical exposure step for proportionally discharging the illuminated areas on an image basis, and a development step wherein the surface of the thermoplastic layer is heated and thus allowed to deform by interaction of surface tension forces and the remaining charge forces. Advantageously, the frost images formed may be viewed directly with simple optical techniques because of the light-scattering character of the deformed surface. They may therefore also readily be displayed by simple optical projection techniques as by transmission of light through the deformed surface, or as in microfilm readers, by reflection of light therefrom. The frost process not only reproduces line images as readily as the relief process but, additionally, the frost images beneficially exhibit solid area coverage and continuous or smoothly graded tone response. Therefore, the frost imaging process has highly desirable versatility, being suited to recording or for storage of data, as well as for performing any of the copying and photographic functions normally associated with xerographic processes, and additionally not requiring the application of liquid or powder toner or developer materials vital to many types of conventional xerography methods. The recorded images generated by the simpler of conventional frost processes are normally "negative" replicas of the original object, i.e., the frost appears at the areas of the latent image not discharged by object light, so that bright areas of the actual object appear dark in the developed image when viewed or read out by simple and inexpensive methods, and vice versa.
More specifically, one common prior art frost recording method uses three primary steps, the first providing uniform charging of a thermoplastic insulating layer surface by actuation of a conventional corona discharge device or corotron. This is followed by exposure of a photoconductor layer associated with the thermoplastic layer with the optical image to be recorded and then by final fixing or development of the image on the insulator surface by substrate or surface heating and then cooling. In such a conventional process, the surface potential of the insulating layer must be high, being generally of the order of 200 to 500 volts with respect to ground, a potential level which is achieved only by applying a unidirectional potential of 8 to 10 kilovolts with respect to ground to the corotron charging electrode for an insulating thermoplastic layer of commercially acceptable thickness. Even higher voltages are required for thicker layers, since the threshold voltage at which frosting obtains is generally proportional to the square root of the thickness of the insulator layer thickness. After charging, the configuration is exposed to a white light image, heavily discharging the most highly illuminated areas. The image is then developed by application of sufficient heat energy to bring the thermoplastic surface at the latent image rapidly almost to its melting point. Upon cooling, the frost image forms as a "negative" replica of the original object image, as previously mentioned. By the use of complex additional elements to the reproducing apparatus for performing additional or modified steps, such as by off-axis (oblique) illumination, it is possible to reproduce the more desirable "positive" image, i.e., a recorded image in which the frost appears at those areas of the latent charge image actually discharged by light, so that bright areas of the object actually appear light in the developed image when viewed or read out by simple and inexpensive methods, and vice versa.
High voltages such as those conventionally used in the charging mode of operation of conventional thermoplastic recording apparatus are well known to be dangerous to the operator; other disadvantages accrue to their use, such as the increased tendency of the circuits involved and the parts of the corotron to be unreliable and to be short lived, even failing catastrophically after only short service. Such failures may represent a fire hazard, or may otherwise cause damage to the recording medium itself which is particularly disadvantageous in data storage systems where the mechanism is in a form suitable for recycling of the thermoplastic surface after erasure of stored data. Aging of the thermoplastic medium is well known to be undesirably accelerated by many cycles of high voltage charging and demonstrates itself by a gradual increase in melting temperature. The material additionally tends to stiffen so that the desired frost deformation is increasingly difficult to form. The aging mechanism may not be fully understood, but may in fact be connected with structural changes producing molecules of greater molecular weight. Also, spectrographic tests indicate that oxidation of the thermoplastic layer may occur. Accordingly, it is seen that conventional frost thermoplastic reproduction or storage techniques, while filling a long felt need in the industry, generally have certain disadvantages, a primary disadvantage being concerned with the requirement for the use of a relatively high charging potential.